New Ways to Produce Geothermal Power at Lower Temperatures

Investments by the U.S.
Department of Energy (DOE) are helping to produce geothermal power at
increasingly lower
temperatures. While traditional
geothermal energy relies on geologically rare hot spots in the subsurface, new
technologies
are making inroads with
economics that could bring valuable returns on geothermal investment in the
near-term.

Geothermal resources are
reservoirs of hot fluid (brine) that exist below the Earth's surface. Wells can
be drilled into these
underground reservoirs to tap
steam and very hot brine that can then be brought to the surface to generate
electricity. In the
U.S., most conventional
geothermal reservoirs are located in the western states, which enjoy a
predominance of geological
anomalies that allow for
relatively shallow and hot geothermal reservoirs to be accessed economically,
creating electricity via
turbine-generators (TG). Traditionally,
these hot zones have been the main focus of geothermal development, as these
resources can be directly used
in a TG set via dry-steam or flash-steam technologies.

The development of binary
technology, however, has allowed for an expansion of the temperature range of
resources
capable of producing
electricity. In these systems, low to moderately heated geothermal fluid (below
300°F) and a secondary
(hence, "binary")
fluid, with a lower boiling point than water, pass through a heat exchanger,
where the heat causes the
secondary fluid to flash to
vapor, driving the TG.

The DOE's Geothermal
Technologies Office (GTO) develops and deploys a portfolio of innovative
technologies for domestic
power generation. Within this
office is the low temperature and co-produced resources subprogram that
conducts research
and demonstration projects in
partnership with industry that will lead to advanced geothermal energy use and
electricity
production from these lower
temperature fluids. But, to truly capitalize on lower temperature resources,
the "value" of
produced fluid must be
increased.

There are a number of ways to
improve the value of geothermal fluid. One is through "cascaded"
technologies, in which the
geothermal fluid cascades from
the highest available temperature for creating electricity down through direct
use applications
that require successively less
heat, including spas, industrial processes and snow melting – each with
distinct temperature
ranges. The GTO is helping to
develop this process through a number of projects, including one in partnership
with the
Surprise Valley Electrification
Corp (SVEC). The SVEC project will produce over 3 MW of power from geothermal
fluid which
will then be used for
aquaculture and irrigation purposes.

Another improvement is the
harvesting of valuable materials from geothermal fluid. In addition to using
the heat to create
power, another project partner,
Simbol Materials, is developing techniques to profitably extract strategic
minerals from brines
via new "geothermal
mining" technologies. The project is validating improved lithium
extraction techniques to transform
mined materials into saleable
products cost-effectively.

The best way of increasing the
value of brines is by improving the efficiency and reducing the cost of power
generation
technologies. The DOE's Pacific
Northwest National Laboratory is in the process of developing microporous
metal-organic
solids as the primary heat
carrier and heat transfer medium that could increase power generation at binary
plants by 15
percent.

Finally, by using water
produced in oil and gas and mining processes, geothermal developers can
leverage existing
infrastructure and greatly
reduce the cost of development, conversely increasing the value of the fluid
stream. ElectraTherm,
another industry partner, is
demonstrating this waste heat-to-power technology at Florida Canyon Mine in
Imlay, Nev. by
using existing groundwork for a
fraction of the cost of new development. This project developed an optimized
solution for
power generation using
co-produced geothermal fluids that will produce a constant 75kW of electricity.